1. Field of the Invention
The present invention relates to an image pickup system and an image processing program, and more particularly to an image pickup system and an image processing program which reduce random noise and the like in color signals due to an image sensor system.
2. Description of the Related Art
Generally noise components are contained in digitalized signals obtained from an image sensor and its analog circuit and A/D converter, and these noise components can be broadly classified into fixed pattern noise and random noise.
The fixed pattern noise is typically represented by defective pixels and mainly originates in the image sensor.
On the other hand, the random noise is generated in the image sensor and the analog circuit and has characteristics similar to the characteristics of white noise.
In regard to the latter of these, i.e., random noise, Japanese Patent Application Laid-open No. 2001-157057 discloses a technique in which a noise amount N is formulated by a function N=abcD where reference symbols a, b, and c denote statically given constant terms and the signal level D is a converted into a density value, the noise amount N is estimated with respect to the signal level D from this function, and the filtering frequency characteristics are controlled based on the estimated noise amount N. As a result, adaptively noise reduction processing is performed with respect to the signal level.
Also, Japanese Patent Application Laid-open No. 2001-175843 as another example discloses a technique in which the input signals are separated into luminance signals and color difference signals, then edge intensity is determined based on these luminance signals and color difference signals, and a smoothing process is carried out in the color difference signals for regions other than the edge portions. Thus, color noise reduction processing is carried out in flat regions.
However, since the luminance noise amount varies dynamically according to factors such as the temperature at the time of shooting, the exposure time, the gain and the like, a technique using static constant terms such as that described in Japanese Patent Application Laid-open No. 2001-157057 cannot formulate into a function that matches the noise amount at the time of shooting and lacks accuracy in estimating the noise amount. Furthermore, although the filtering frequency characteristics are controlled from the noise amount, this filtering is carried out uniformly without distinguishing between flat regions and edge regions, and therefore the edge deteriorate in regions estimated that the noise amount is large from the signal level. That is, there are problems in that there is no capacity for handling processing that distinguishes the original signals and the noise and there is poor maintainability of the original signals. Further still, the technique described in the aforementioned application has no capacity for handling color noise produced between color signals.
Also, although the technique described in Japanese Patent Application Laid-open No. 2001-175843 carries out a smoothing process on color difference signals in flat regions except for the edge regions, this smoothing process is carried out fixedly. However, since the color noise amounts vary depending on the signal levels, optimal control of this smoothing process cannot be achieved. Thus, there is a likelihood that color noise components will remain and the original signals will deteriorate.
The present invention has been devised in light of these circumstances and it is an object thereof to provide an image pickup system and an image processing program capable of reducing color noise with high accuracy and optimized to the shooting conditions and producing high quality images.